The present invention provides an improved ultrasonic holography or other ultrasonic imaging process that accurately forms the phase and amplitude information of the hologram in a manner that renders the unit insensitive to environment vibrations, and provides long maintenance free functioning lifetime. Specifically, the improved ultrasonic hologram detector component forms an ultrasonic hologram on the surface of a liquid that results from the deformation of the surface. This is due to the reflection of an ultrasound energy profile of a combination of an xe2x80x9cobject wavexe2x80x9d that passes through the object and that of a xe2x80x9creference wavexe2x80x9d that is directed to the surface at an off axis angle from the xe2x80x9cobject wavexe2x80x9d.
The central element field of holography is fulfilled by combining or interfering an object wave or energy with a reference wave or energy to form an interference pattern referred to as the hologram. A fundamental requirement for the forming of the hologram and the practice of holography is that the initial source of the object wave and reference wave or energy are coherent with respect to the other wave. That is to say, that all parts of both the object wave and the reference wave are of the same frequency and of a defined orientation (a fixed spatial position and angle between the direction of propagation of the two sources). When performing holography the object wave is modified by interference with structure within the object of interest. As this object wave interacts with points of the object the free-dimensional features of the object impart identifying phase and amplitude changes on the object wave. Since the reference wave is an unperturbed (pure) coherent wave, its interference with the object wave results in an interference pattern which identifies the 3-D positioning and characteristics (ultrasonic absorption, diffraction, reflection, and refraction) of the scattering points of the object.
A second process, (the reconstruction of the hologram) is then performed when a coherent viewing source (usually light from a laser) is transmitted through or reflected from the hologram. The hologram pattern diffracts light from this coherent viewing or reconstructing source in a manner to faithfully represent the 3-D nature of the object, as seen by the ultrasonic object wave.
To reiterate, to perform holography coherent wave sources are required. This requirement currently limits practical applications of the practice of holography to the light domain (e.g., a laser light) or the domain of acoustics (sometimes referred to as ultrasound due to the practical application at ultrasonic frequencies) as these two sources are currently the only available coherent energy sources. Thus, further references to holography or imaging system will refer to the through transmission holographic imaging process that uses acoustical energies usually in the ultrasonic frequency range. In the practice of ultrasound holography, one key element is the source of the ultrasound, such as a large area coherent ultrasound transducer. A second key element is the projection of the object wave from a volume within the object (the ultrasonic lens projection system) and a third is the detector and reconstruction of the ultrasonic hologram into visual or useful format.
Although other configurations can be utilized, a common requirement of the source transducers for both the object and reference waves is to produce a large area plane wave having constant amplitude across the wave front and having a constant frequency for a sufficient number of cycles to establish coherence. Such transducers will produce this desired wave if the amplitude of the ultrasound output decreases in a Gaussian distribution profile as the edge of the large area transducer is approached. This decreasing of amplitude reduces or eliminates the xe2x80x9cedge effectxe2x80x9d from the transducer edge, which would otherwise cause varying amplitude across the wave front as a function distance from the transducer.
In the process of through transmission ultrasonic holographic imaging, the pulse from the object transducer progresses through the object, then through the focusing lens and at the appropriate time, the pulse of ultrasound is generated from the reference transducer such that the object wave and reference wave arrive at the detector at the same time to create a interference pattern (the hologram). For broad applications, the transducers need to be able to operate at a spectrum or bandwidth of discrete frequencies. Multiple frequencies allow comparisons and integration of holograms taken at selected frequencies to provide an improved image of the subtle changes within the object.
A hologram can also be formed by directing the object wave through the object at different angles to the central imaging axis of the system. This is provided by either positioning or rotating the object transducer around the central axis of transmission or by using multiple transducers positioned such that the path of transmission of the sound is at an angle with respect to the central axis of transmission.
With a through-transmission imaging system, it is important to determine the amount of resolution in the xe2x80x9czxe2x80x9d dimension that is desirable and achievable. Since the holographic process operates without limits of mechanical or electronic devices but rather reconstructs images from wave interactions, the resolution achievable can approach the theoretical limit for the wavelength of the ultrasound used. However, it may be desirable to limit the xe2x80x9czxe2x80x9d direction image volume so that one can xe2x80x9cfocusxe2x80x9d in on one thin volume slice. Otherwise, the amount of information may be too great. Thus, it is of value to develop a means for projecting a planar slice within a volume into the detector plane. One such means is a large aperture ultrasonic lens system that will allow the imaging system to xe2x80x9cfocusxe2x80x9d on a plane within the object. Additionally, this lens system and the corresponding motorized, computer controlled lens drive will allow one to adjust the focal plane and at any given plane to be able to magnify or demagnify at that z dimension position.
The image is detected and reconstructed at the detector. Standard photographic film may be used for the recording of light holograms and the 3-D image reconstructed by passing laser light through the film or reflecting it from the hologram pattern embossed on the surface of an optical reflective surface and reconstructing the image by reflecting light from the surface. However, there is no equivalent xe2x80x9cfilmxe2x80x9d material to record the intricate phase and amplitude pattern of a complex ultrasonic wave. One of the most common detectors uses a liquid-air surface or interface to record, in a dynamic way, the ultrasonic hologram formed. The sound energy at the frequency of ultrasound (above range of human hearing) will propagate with little attenuation through a liquid (such as water) but cannot propagate through air. At these higher frequencies (e.g., above 1 MHz) the ultrasound will not propagate through air because the wavelength of the sound energy is so short (xcex(wavelength)=v(velocity)/f(frequency)). The density of air (approximately 0.00116 g/cm3) is not sufficient to couple these short wavelengths and allow them to propagate. On the other hand the density of a liquid (e.g., water) is a favorable media to couple and propagate such sound. For example, the velocity of sound in air is approximately 330 meters/second whereas in water it is approximately 1497 meter/second (room temperature). Thus, for water, both the density (1 g/cm3) and the wavelength (xcx9c1.48 mm at 1 MHz) are significantly large such that ultrasound can propagate with little attenuation. Whereas, for air both the density (0.00116 g/cm3) and wavelength (0.33 mm at 1 MHz) are sufficiently small such that the energy at these ultrasonic frequencies will not propagate.
Thus, when ultrasound propagating in a liquid encounters a liquid-air interface the entire amount of the energy is reflected back into the liquid. Since ultrasound (or sound) propagates as a mechanical force it is apparent that the reflection (or changing direction of propagation) will impart a forward force on this liquid air interface. This force, in turn, will distort the surface of the liquid. The amount of surface distortion will depend upon the amplitude of the ultrasound wave at each point being reflected and the surface tension of the liquid. Thus, the pattern of the deformation is the pattern of the phase and amplitude of the ultrasonic wave.
It is in this manner that a liquid-air interface can be commonly used to provide a near realtime recorder (xe2x80x9cfilm equivalentxe2x80x9d) for an ultrasonic hologram. The shape of the surface deformation on this liquid-air detector is the representation of the phase and amplitude of the ultrasonic hologram formed by the interference of the object and reference ultrasonic waves.
The greatest value of the ultrasonic holographic process is achieved by reconstructing the hologram in an usable manner: usually in light, to make visible the structural nature of the initial object. In the case of a liquid-air interface, the reconstruction to achieve the visible image is accomplished by reflecting a coherent light from this liquid-air surface. This is the equivalent process to reflecting laser light from optically generated hologram that is embossed on the surface of a reflecting material (e.g., thin aluminum film).
The reflected light is diffracted (scattered) by the hologram to diffracted orders, each of which contains image information about the object. These diffracted orders are referred to as xc2x1n th orders. That part of the reconstructing light that does not react with the hologram is referred to as zero order and is usually blocked so that the weaker diffracted orders can be imaged. The higher the diffracted order the greater the separation angle from the zero order of reflected light.
Once reconstructed, the image may be viewed directly, by means of a video camera or through post processing.
Ultrasonic holography as typically practiced is illustrated in FIG. 1. A plane wave of sound (1a) (ultrasound) is generated by the object (large area) transducer (1) (U.S. Pat. No. 5,329,202). The sound is scattered (diffracted) by structural points within the object (2). The scattered sound is from the internal object points that lie in the focal plane (2a) are focused (projected) into the ultrasonic hologram plane (6). The focusing takes place by use of ultrasonic lens (3) (U.S. Pat. No. 5,235,553) which focuses the scattered sound into a hologram detector surface (6) and the unscattered sound into a point (4). The lens system also allows the imaging process to magnify the image or change focus position (U.S. Pat. No. 5,212,571). Since the focus-point of the unscattered sound (4) is prior to the holographic detector plane (6), this portion of the total sound again expands to form the transparent image contribution (that portion of the sound that transmitted through the object as if it were transparent or semitransparent). In such an application, an ultrasound reflector (5) is generally used to direct the object sound at a different angle (preferably vertically to allow for the holographic detector to have a surface parallel to ground to avoid gravity effects), thus impinging on horizontal detector plane usually containing a liquid which is deformed by the ultrasound reflecting from the liquid-air interface. When the reference wave (7) and the object wave are simultaneous reflected from this detector, the deformation of the liquid-air interface is the exact pattern of the ultrasonic hologram formed by the object wave (1a combined with 2a) and the xe2x80x9coff-axisxe2x80x9d reference wave (7).
This ultrasonic hologram formed in the holographic detector (6) is subsequently reconstructed for viewing by using a coherent light source (9), which may be passed through an optical lens (8), and reflected from the holographic detector surface (U.S. Pat. No. 5,179,455). This reflected coherent light contains two components. These are A: The light that is reflected from the ultrasound hologram which was not diffracted by the ultrasonic holographic pattern which is focused at position (10) and referred to as undiffracted or zero order light; and B: The light that does get diffracted from/by the ultrasonic hologram is reflected at an xe2x80x9coff-axisxe2x80x9d angle from the zero order at position (11) and referred to as the xe2x80x9cfirst orderxe2x80x9d image view when passed through a spatial filter (12). It is noted that this reconstruction method produces multiple diffraction orders each containing the ultrasonic object information. Note also both + and xe2x88x92 multiple orders of the diffracted image are present and can be used individually or in combinations to view the optical reconstructed image from the ultrasonically formed hologram by modifying the spatial filter (12) accordingly.
The practice of ultrasonic imaging is in the industrial or hospital settings where insensitivity to vibrations and long term stable operations are important to the successful use of the system. It is further a requirement that the detector method be able to image subtle structures within the object being imaged and to often provide individual frames on images as fast as 120 per second. This need is particularly strong, for example, for breast cancer screening techniques that now utilize invasive mammography (providing the patient with a dose of radiation from XRay imaging) and yet do not have high quality images that lend a sense of three dimensional structure to breast tissue.
Therefore, there is a need in the art to improve resolution characteristics of transmissive ultrasonic imaging, ability to operate in environments experiencing vibrations and to perform over extended periods of time without service or degradation of image quality.
The present invention provides an ultrasonic hologram detector apparatus comprising:
(a) a rigid housing component describing a cavity defined by a floor component composed of rigid polymeric material having an upper surface forming a first plane and lower surface forming a parallel second plane, and rigid side elements attached to the base, wherein the cavity defines an enclosed space of a dimension of the upper surface of the floor component and the rigid side elements of from about 1 cm to about 5 cm in height, wherein the distance between the upper surface and the lower surface of the floor component is from about 5 mm to about 7.5 mm;
(b) a layer of detection fluid contained within the cavity, wherein the detection fluid has a thickness of from about 0.2 mm to about 0.50 mm whereby surface or horizontal vibration waves at the frequencies experienced in buildings cannot be propagated; wherein the detection fluid has a surface tension of from about 12 dynes/cm to about 19 dynes/cm, wherein the detection fluid has a Kinematic viscosity of from about 1 cs to about 20 cs; and
(c) an inert gas filling a space in the cavity above the detection fluid.
Preferably, the inert gas is selected from the group consisting of nitrogen, helium, argon, and combinations thereof. Most preferably, the thickness of the detection fluid within the cavity is a multiple of xc2xc wavelengths at the frequency being used for imaging, whereby internal reflections within the detector liquid are minimized.
Preferably, the ultrasonic hologram detector further comprises a top of the cavity composed of optically transparent material. Most preferably, the optically transparent material is glass. Most preferably, the glass is formed into an optical focusing element (a lens) or an optically transparent sealing cover to the cavity. Most preferably the optically transparent top cover further comprises a heating element associated and in contact with the cover.
Preferably, the floor component material is characterized by (i) attenuation of ultrasonic energy of less than 8% per cm of the material, (ii) a velocity of shear waves mode ultrasound propagation that results in acoustical impedance such that a reflection shear wave mode is less than 2% at the boundary of the floor component material and transmission liquid medium, and (iii) reflection of a longitudinal mode of propagation for angles of incidence of greater than 60 degrees from normal to an interface with the ultrasound liquid transmission medium, whereby a velocity for longitudinal mode of greater than 1730 m/sec. Most preferably, the transmission liquid medium is water, wherein the floor component material has an ultrasonic shear wave impedance of from about 1170 to about 1900 and an ultrasonic impedance (velocity times the density) of greater than 1200 but less than 2000. Such selected characteristics will result in a shear wave mode reflection {(zSxe2x88x92ZW)/(zS+zW)){circumflex over ( )}2 where zS is the impedance of the shear wave in the floor component and zW is the impedance of water} of less than 2.1% at the floor component/water media interface. Most preferably, the velocity for longitudinal mode in the floor component is greater than 1730 and less than 2700 m/sec. Preferably, the detection fluid has vapor pressure of from about 1 torr to about 5 torr. Preferably, the detection fluid has a velocity of sound of less than 1,000 m/Sec. Preferably, the detector fluid is a fluorinated (in place of hydrogen) organic compound having from 3 to 10 carbon atoms in a straight or branched chain or an aqueous solution with reduced surface tension additives, wherein the viscosity of the detector fluid is such that the deformation will be formed on the liquid surface in less than 200 micro seconds and yet will be quiescent within less than 0.0083 or ({fraction (1/120)}) seconds. Most preferably, the velocity of the detection fluid is approximately that of water (1497 m/sec) when surface tension reduction additives are used.